The invention relates to control circuits, and more particularly, to a gain control circuit, and a variable gain amplifier using the same.
FIG. 1 shows a conventional variable gain amplifier. As shown, currents through a differential pair composed of transistors T9 and T10 are controlled by the voltage V CTRL, and thus, the current IL through LD can be adjusted. As follows, the current IL can be obtained by equation 1, wherein gm is the gain of the variable gain amplifier 50, VRF is an input voltage and VT is a temperature parameter.
                              Irf          =                      gm            ×                          V              RF                                      ⁢                                  ⁢                              I            L                    =                      Irf            ⁡                          (                              1                +                                  tanh                  ⁢                                                                          ⁢                                                            V                      CTRL                                                              V                      T                                                                                  )                                                          (        1        )            
FIG. 2 and FIG. 3 show relationships between the gain control voltage VCTRL and the gain gm of the amplifier 50 respectively. As shown, the relational curve RC of the gain control voltage VCTRL and the gain gm is not linear in the range of 0˜20 db. A gain control circuit is required to generate a corresponding gain control voltage according to an external control voltage to obtain relational curve RC for the amplifier 50, wherein there is a log-linear relationship between the external control voltage and the gain of the amplifier 50.
Further, current IL relates to the temperature parameter VT, wherein
            V      T        =          kT      q        ,k is Boltzmann constant, T is absolute temperature, and q is electron charge.
In view of this, current switching characteristics of the variable gain amplifier 50 changes with temperature, such that gain of the amplifier 50 is affected by temperature and log-linear relationship between an external control voltage and gain of amplifier gain 50 cannot be obtained.